1. Field of the Invention
This invention relates to a color image processing apparatus in which undercolor removal is performed in a digital color copier or the like.
2. Description of the Prior Art
Conventional color image processing apparatus use inks of four colors, i.e., yellow, magenta, cyan and black, and obtain a full color image by the subtraction color-mixing method.
For representing a black region on a manuscript, there are, in general, two methods: a method in which black is represented by printing, with overlapping, inks of three colors, i.e., yellow, magenta and cyan; and a method in which black is represented by printing black ink instead of printing inks of three colors, yellow, magenta and cyan. One or the other of these methods is selectively used, depending on the situation.
Now, the undercolor removal ("UCR") method used in printing or the like is a method in which the smallest, or minimum, from among the respective amounts of yellow, magenta and cyan inks is determined, the actual amount of each ink to be used is reduced according to the determined minimum amount, and the total amount of the reduction (the amount of inks thus reduced) is replaced by black ink.
Thus, it becomes possible to reproduce a black which has an optical density higher than that of a black produced by printing, with overlapping, yellow, magenta and cyan inks. Moreover, it is possible to reduce the amount of consumption of yellow, magenta and cyan inks.
However, when all of the black of a region is replaced by black ink (100% UCR), half-tone images become rough. Hence, 100% UCR can not be put into practice, and yellow, magenta and cyan inks are printed, with overlapping, on a part of the above-described black region.
When yellow, magenta and cyan inks are printed with overlapping on a part of the black region as described above, there are problems in that yellow, magenta and cyan inks remain also in a black-letter region, and the influence of imperfect registration and dispersion of inks can become remarkable, deteriorating the quality of black letters.
Japanese Patent Provisional Publication No. 59-205,876 (1984), counterpart of U.S. patent application Ser. No. 07/058,625, filed June 4, 1987, a continuation of U.S. patent application Ser. No. 06/608,238, filed May 8, 1984, has disclosed an apparatus in which color-separated R, G and B signals are converted into four-primary-color signals, Y, M, C and K, and subsequently it is judged, using only the K signal, whether an image region in question is a high-resolution region, such as a letter image or the like, or a region which does not require high resolution, such as a photographic image or the like. Then, when it is judged a high-resolution region, all the four colors, Y, M, C and K, are output in binary numbers, and when it is a photographic region, all the four colors, Y, M, C and K, are output in multiple (multibit) numbers as they are.
There is also another apparatus in which image-region identification is independently performed for each of the Y, M, C and K signals to determine whether each color should be output in binary numbers or in multiple (multibit) numbers as it is, according to the identification result.
However, when image-region identification is independently performed for each of the Y, M, C and K signals, the identification result sometimes is different for each color, and the resulting output image tends to have uneven color, blurred color or the like.
Moreover, when image-area identification is performed using only K signals, the following inconvenience, for example, can occur. Suppose that an object picture element is represented by each of four 8-bit digital signals Y, M, C and K, and, for example, Y=255, M=255, C=150 and K=min. (Y, M, C)=150, i.e., a blackish red color. When the object picture element is judged to be a picture element in a letter region according to the status of the K signal of a neighboring picture element thereof, all of the Y, M, C and K signals are converted into binary numbers. Hence, if the threshold value for converting into a binary number is assumed to be 128 which is an intermediate level, binary-number output of the object picture element becomes Y=M=C=K=1 (level 255 ), and the picture element is replaced by a completely dense black.
U.S. Pat. No. 4,700,399 and U.S. patent application Ser. No. 07/173,654, filed Mar. 25, 1988, both assigned in common with the present invention, disclose other image processing apparatus, but those apparatus leave room for improvement in respect of the objects of the present invention.